1. Field of the Invention
The present invention generally relates to a focusing method for an electron microscope. In particular, the invention concerns a transmission electron microscope provided with afacility for photographic recording which allows observation and recording of a specimen image to be carried out in a much facilitated manner while assuring an improved image quality by virtue of an incorporated automated focusing system so arranged as to suppress focal deviations within a permissible range for a selected magnification, particularly in a range of moderate and low magnifications.
2. Description of the Prior Art
Photographic recording of a specimen image produced by a transmission electron microscope is rather a troublesome job requiring high skill. It is certainly troublesome to bring the image under observation to a properly focussed state as desired with the aid of the image produced on a viewing fluorescent screen which is usually poor in resolution. This can be explained by the fact that the magnification at which the image is produced on the screen is lowered by 10 to 30% when compared with the magnification at which the image is projected onto a photographic film and that resolution of the film is on the order of 10 to 40 .mu.m while that of the fluorescent, screen is on the order of 50 to 100 .mu.m. Under the circumstance, even the sharp and clear image viewed on the screen will often result in a blurred image when recorded photographically on the film.
By the way, a biological specimen is susceptible to injury by electron beam, whereby the intrinsic structure of the specimen may be destroyed to make it impossible to obtain desired information from the specimen, as is known in the art. For this reason, selection of the field of view is effected in the state where energy or density of the electron beam is considerably reduced. Of course, the photographing itself will certainly involve substantially no damages to the biological specimen, because of an extremely short time as required. However, since the specimen imaging may not be effected in a satisfactory manner unless fine structure of the specimen in concern can be observed, the specimen undergoes necessarily irradiation with the electron beam of a high density for a relatively long time in order to produce a bright and clear image on the fluorescent screen, which in turn means that the biological specimen is subjected to appreciable injuries.
In reality, the troublesome and time consuming procedure required for the focusing has provided a great obstacle in realizing the automated photographic recording of the microscopical image. More particularly, automation of the photographic recording system is at present accomplished so far as the exposure or light meter capable of detecting at high precision the energy level of the electron beam incident on the fluorescent screen, a shutter mechanism for assuring a proper exposure in dependence on the indication output from the light meter, an automatic film feed and a data printer for printing data like the magnification as employed, the frame number and the like are concerned. Besides, there has been already developed an automated view field selecting system in which a specimen stage adapted to be driven by an electric motor is combined with a central processing unit or computer so that portions of a specimen to be observed can be stored and read out. With this system, a number of view fields or points to be observed are previously stored in a memory equipment, wherein the fields of view are selectively read out one-by-one or successively for the observation and/or recording. In contrast, the imaging or focusing of the specimen image relies utterly on the manipulation by operator himself, taking a lot of time providing a difficulty in attaining the rapid recording as a whole in spite of the automation of the relevant mechanisms described above. Various attempts have heretofore been made for accomplishing the automatic focussing. For example, it is conceived that an image on the fluorescent screen is converted into electric signals which are then utilized in an electronic unit for automatically determining whether the focusing or specimen imaging is accomplished to a satisfactory degree. However, such system is not only complicated and expensive but also incapable of presenting reliable measures because of poor availability of the signals and other causes. Such being the circumstance, the automatic focusing system for taking picture of the microscopic image is at present far from practical application.